Positron emission tomography (PET) is a medical imaging modality that uses positron emitting radionuclides attached to specific molecules that are introduced into the body of an animal and tracked by an imaging system. This provides sensitive assays of a wide range of biological processes, e.g., processes related to cancer or other diseases associated with alterations in the functional metabolism of cells in the body. Tracking is possible because the radionuclides emit high-energy photons that are detected with high spectral and temporal resolution by imaging system detectors. A principal drawback of existing PET technology, however, is its relatively poor spatial resolution (e.g., 1-2 mm) making unambiguous localization of signals difficult in many cases.
On the other hand, alternative imaging modalities such as, but not limited to, magnetic resonance imaging (MRI) provide extremely detailed anatomical information with high spatial resolution, e.g., as good as 20 microns. Moreover, innovative techniques in methods such as MRI have led to a remarkable increase in the range of physiological information that can be obtained during an imaging session in addition to conventional structural MRI, and this information has further helped to improve diagnosis and treatment.
As nonlimiting examples, diffusion-weighted MRI (DWI) has become an established technique for the early diagnosis of acute stroke. Perfusion-weighted MRI (PWI) is an evolving MRI technology for studying cerebral hemodynamics and blood flow, with the creation of hemodynamic maps of cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT) or time to bolus peak (TTP). As another example, functional MRI (fMRI) can image hemodynamic and metabolic changes that are associated with human brain functions, such as vision, motor skills, language, memory, and mental processes; it is in use for the localization of visual, motor, and somatosensory responses in surgery of tumors, as well as for elucidation of brain function and metabolism altered by pathologies such as stroke, multiple sclerosis, and Alzheimer's disease.
Along with the development of novel MRI techniques, a current trend is toward the use of higher magnetic fields (e.g., 3 T and higher), mainly driven by the expected gain in the signal-to-noise ratio (SNR) that roughly correlates with the field strength, and that may be used to obtain even higher spatial resolution and contrast, and to reduce MRI scanning time. Higher fields also give rise to several challenges (stronger susceptibility effects, increased physiological noise and RF power deposition, etc.), but they have been proven to facilitate study of brain biophysics and biochemistry (metabolic cycles, perfusion, neuronal architecture, etc.), and of other organ systems as well (breast, musculoskeletal, etc.). Although some cutting-edge material is still under development, and additional time might be required until this technology is commonly available, most of these methods can be applied on most state-of-the art MRI scanners.
Given the complementarity of the information provided by PET and methods such as MRI, as well as the level of performance that has been recently achieved by the two separate modalities, many expectations have arisen recently from the conceptual possibility to acquire simultaneous PET and MR images, and thus 1) obtain an immediate anatomical localization of the PET signal as well as 2) observe the simultaneous dynamic correlation between the functional behavior of tissues and their anatomy. Also, further discoveries and consequent improvements in clinical practice can originate from the integration of more advanced MR measurements, such as but not limited to dynamic contrast enhanced MR, diffusion-tensor imaging, functional MRI, MR spectroscopic imaging, neuronal tract tracing, cell trafficking, and paramagnetic contrast agents, with tracer kinetic experiments utilizing radiolabeled probes that are targeted to specific aspects of the biology of interest.
Despite these expectations, however, such combined PET and MRI scanners are not readily available in the art. As one reason, it is very challenging to build PET electronics that can fit compactly into the small bore of an MRI system, while at the same time not degrade in the presence of the extremely high static and dynamic fields of the MRI system. At the same time, it is easy for the PET detectors to inject noise into the extremely sensitive RF pickup of an MRI system. For example, there are several ways in which the two imaging modalities can interfere with each other, causing artifacts and degradation in image quality, including: the presence and the operation of the PET instrumentation inside or close to the MR imaging region perturb the homogeneity of the BO field and the linearity of the gradient fields in the MR system; radiofrequency interference may occur between the MR transmit/receive coils and the electronics of the PET system; and the placement of materials inside the static and switching magnetic fields of the MR magnet may lead to susceptibility artifacts, eddy currents, and other effects, which may compromise the operations of the PET+MRI system.